2 ways you can darken material in SU, 1 to use B(bucket) to open your material window and click on ‘‘edit’’ and then choose the HSL to ‘‘dim down’’ the brightness.
another way is to set the sun light in shadow.
If everything is fine on screen but the print out still looks dark I think you probably make a final brightness adjust in other software like PS or PPT.

You can choose one directory, probably the latest version, and copy it to a common location in your user path like:%USERPROFILE%/Documents/SketchUp/Library/Layout/PatternFills
(… or whatever path makes sense to you.)

Then click in the address bar and copy the new user path, and (ALT+TAB) switch to Layout.Edit (menu) > Preferences (dialog) > Folders (panel)
Click the + button under Pattern Fill Images (bottom box of the 3,) and paste in the new user path.
(Do this for any Layout version you are using.)

Then use a graphics editor to edit the png patterns in the user path.

As you compile a library of user custom patterns you can share them among your computers or friends via DropBox, OneDrive, etc,